The present invention relates to a bootstrap circuit used for a shift register circuit or a buffer circuit. More specifically, the present invention relates to a bootstrap circuit in which a voltage that is equal to or lower than a power source voltage for the circuit is applied between a source and a drain of all transistors forming the circuit.
Shift register circuits and buffer circuits are widely used for generating a scan signal and/or driving a matrix array in matrix-type display devices, image sensors, or semiconductor memory devices.
A push-pull output circuit is typically used for an output stage of a shift register circuit or a buffer circuit of the type described. For example, an example is given with an output circuit employing an N-channel transistor. A voltage Vgs between a gate and a source gradually decreases as an output voltage is raised in a transistor on the higher potential side among the transistors forming the push-pull output circuit. The transistor is turned off when the voltage Vgs becomes smaller than Vt, that is, Vgs&lt;Vt where Vt is a threshold voltage of the transistor. Therefore, (Vgs-Vt) is the lower limitation appeared on the amplitude waveform of the output voltage across the output circuit. Such output circuit has a limitation of fast operation.
Taking this into consideration, output circuits using a bootstrap effect have been proposed. The output circuit of this type can provide the output voltage that is equal to the power source voltage Vdd and is applicable to fast operation. Therefore, this output circuit is widely used for circuits that require performance at a high speed.
As apparent from the description below, the conventional output circuits using the bootstrap effect have the problem that it is necessary:
(a) to design the circuit to restrict the bootstrap effect;
(b) to lower the power source voltage for the circuit; and
(c) to provide a circuit configuration requiring no reset transistor.